Iain Duncan Smith, the work and pensions secretary (left) and a disability campaigner protesting outside the Houses of Parliament in June this year.

The Department for Work and Pensions (DWP) has refused to comply with a Freedom of Information request sent by BuzzFeed News asking for the death rate among people found "fit for work", saying that finding the four relevant numbers in its data was too complicated.

The DWP has been dogged for more than a year by claims that thousands of people have died after having their disability benefit removed. The impact of a fit-for-work verdict on a vulnerable person was starkly revealed in September of this year, when a coroner declared that one man had killed himself as a direct result of being found fit for work.

In an attempt to establish the death rate of people who had been found fit for work, BuzzFeed News submitted an FOI request of its own, asking for the "age-standardised mortality rate" (ASMR) for people found fit for work since December 2011. The ASMR is how many people out of every 100,000 die on average every year, taking into account their age. If the DWP kept comprehensive records of its actions then the ASMR would be easy to work out.

However, in case the DWP was not keeping track, BuzzFeed News also asked for the number of people assessed, the number of people who died, and the number of people who had moved from the Employment and Support Allowance (ESA) to other benefits, so that it could work out the mortality rate itself.

The ESA replaced the old disability benefits as part of a sweeping reform of benefits payments led by the work and pensions secretary, Iain Duncan Smith, which began under the last government.

In its FOI response – filed more than a month past the deadline – the DWP said that "we can confirm that the Department does hold some information falling within the description specified", but said that retrieving the information would be too costly. It said that it would take an employee more than 3.5 working days to find the four numbers requested.

Disability rights campaigners protest outside the Houses of Parliament in June of this year.

The DWP's response, although 34 days late, arrived within four hours of BuzzFeed News telling the department's press office that it was planning to write up a story that the DWP had failed to respond to the request. A spokesperson for the DWP would not deny that the two events were connected.

The 2,300 statistic, however, is not informative, because the DWP did not release the total number of people who were found fit for work. It was therefore impossible to tell whether the death rate was higher than would be expected. The journalist and doctor Ben Goldacre described the DWP's response as "full of red herrings" and "a stroke of evil genius [which] confused a lot of people".

On 1 September, BuzzFeed News filed its FOI request. It received an automated response promising a reply by 29 September. After hearing nothing it chased up several times by telephone and by email on 1 October, 13 October, and 2 November.

On the morning of 2 November it told the DWP press office that it would be running a story about their failure to comply with an FOI request and asked for a comment. By the afternoon it had received the FOI response. BuzzFeed News rang the press office again and asked if the timing was connected, which a spokesperson would not deny.

The DWP told BuzzFeed News: "We are committed to being the most transparent government ever and this department answers thousands of FOIs every year, the vast majority within 20 days. We recently published a wide range of information on this subject."

BuzzFeed News has asked the DWP to begin an internal inquiry into their failure to provide the requested information.